BIOLOGY OF THE EARTHWORM

Habits of the Earthworm

Earthworms are to be found in the surface layers of every moist soil, and are especially numerous if the soil is rich in humus. If the surface soil becomes too dry they descend to moister regions, and they pass the winter in an inactive state coiled up in a kind of nest lined with small stones below the surface of the ground. They spend the daytime in the ground, but at night, especially if the surface is damp, they come out and crawl about in search of food. Frequently a worm does not come right out of its burrow but keeps the end of its body in the hole so that it can retire underground quickly if danger threatens. While in this position the free end sweeps round and round in circles in search of food. Each burrow leads vertically down into the soil for several feet, and often ends in a small chamber. Sometimes dead leaves are to be seen sticking out of the entrances to worms ’ tunnels. These leaves have been dragged there by the worms, and probably help to hide the entrance, as well as to keep the burrow from becoming too dry. Other leaves are dragged right into the burrows and used as food.

The worm tunnels partly by force and partly by eating its way through the soil. It is able to obtain nourishment from the small proportion of decayed animal and plant remains that is present in the soil which is swallowed. The soil is ground up by a part of the food canal known as the gizzard, and the finely powdered soil that is passed out of the body forms the little heaps at the entrance to the burrow, which are known as worm casts. The burrow is lined with a layer of this fine soil, stuck together by the fluid secreted by the worm ’s skin.

A worm has no eyes but can tell when it is near the surface, for its front end is sensitive to bright light, which the worm avoids. A worm has no ears either but it is able to perceive vibrations in the soil, and so will not venture from its burrow while a bird is hopping about overhead.

The Importance of the Earthworm to Man

Few people realize what an important part earthworms play in relation to agriculture. By their burrowing activities worms loosen the soil so that the roots of plants can grow down more easily. In addition, the entrance of rain and air into the soil is helped by the burrows, which both drain and aerate the soil. Leaves which have been dragged into the burrows by worms and which have not been eaten, enrich the humus content of the soil when they decay. Gradually, owing to the continual burrowing and formation of worm casts by the worms, the soil throughout a depth of 1 or 2 ft. is brought to the surface, which becomes covered by a layer of finely powdered material. Darwin spent much time in examining the habits of worms, and he calculated the rate at which worm casts accumulate on the surface. He estimated that there are about 53,000 worms in an acre of garden soil, and about half that 28 number in an acre field. He calculated that fresh soil is brought to the surface of a garden at the rate of 3 in. in fifteen years, and that the amount of earth that passes in a year through the earthworms of England is 320,000,000 tons. One result of this raising of soil by worms is that stones lying on the surface are gradually covered up. Darwin cites an example of a field which was covered with lumps of chalk and left undisturbed by man for thirty years. At the end of this time the chalk was found to be 7 in. below the surface.

The External Features of the Earthworm

The body of the worm is roughly cylindrical in shape, except at the hinder end which is flattened from above downwards, and it is marked by transverse grooves into about 150 rings or segments. The long pointed shape of the body, which may be 6 in. long, aids the worm in making its way through the ground. The dorsal surface is darker than the ventral surface, and the whole body is covered by a thin layer of cuticle, through which a blood vessel can be seen running along the dorsal surface. The surface of the worm is moist to the touch owing to a secretion from tiny glands, which is poisonous and so keeps the skin clear from bacteria and fungi. The secretion acts as a lubricant during the worm ’s movement through the soil, and helps to stick the soil particles together in the wall of the burrow. It also maintains the skin in the moist condition that is essential for the respiration of the worm.

On the dorsal surface of the worm are small holes, the dorsal pores, which open in the grooves between the segments and place the interior of the body in communication with the exterior. One dorsal pore occurs in each groove behind the 10th segment. The first segment is called the peristomium and surrounds the mouth, which is overhung by a flap known as the prostomium. The anus is a small opening in the last segment of the body. About a quarter of the way along the body from the anterior end are five or six segments which have a swollen appearance. This is a glandular region called the clitellum, which secretes the cocoon into which the worm lays its eggs.

If a worm is drawn through the fingers a roughness due to small bristles can be felt. These bristles, which are called chrctae, project from the body wall in two double rows on both sides of the body, so that every segment, except the first and last, bears four pairs of chastae. On each side one pair of chaitae occurs laterally, and the other pair lies beneath it nearer the ventral surface.

The only easily visible openings of the various internal organs are those of the male reproductive organs, which occur on the lower side of the 15th segment. They are transverse slits surrounded by fleshy lips.

Movement

The body wall of the earthworm is provided with two layers of muscle, which are an external layer running round the body and an internal longitudinal layer running from end to end. When the worm is crawling over the surface the chajtae in the hinder part of the body are pushed into the ground and the circular muscles in the front part of the body contract; the pressure of the body fluid so set up then causes the front part of the worm to move forwards. Next the chastae of the front part of the body are pushed into the ground and those of the hind part are withdrawn. Finally, by the contraction of the longitudinal muscles and the relaxation of the circular muscles, the hind part is drawn into a new position. Thus by the aid of the chretae the worm moves over the ground by the alternate shortening and lengthening of the body which gives the effect of waves of expansion and contraction passing alternately along the body. It is due to the Sometimes called sefce, but they are quite different from the seta; of insects. projection of the chietae that it is easier to break a worm in two than to draw it out of its burrow.

The Internal Structure of the Worm

The body of Hydra is extremely simple in structure, consisting of two layers of cells, the ectoderm and endoderm, separated by a layer of jelly-like material. The structure of the worm is more complicated, for in addition to these two cell layers there is a double-layered tissue called the mesoderm which lies between them, and which encloses a space called the coelom.

All animals which possess a coelom are referred to as the Ccelomata. The coelom is lined with a layer of cells, one cell in thickness, termed the peritoneum and contains a fluid called the coelomic fluid. The main structure of the worm consists essentially of two tubes, one fitted inside the other. The inner tube is the food tube or alimentary canal and the other one the body wall of the worm. The space between the two tubes is the coelom, and in the worm it is divided into a number of chambers, each corresponding to a segment, by transverse partitions called septa. The middle of the lower edge of each septum is not fixed to the body wall, and the gap thus left allows the coelomic fluid to pass from one segment to another.

The Body Wall

The surface of the body is covered by a thin membrane, the cuticle, which is secreted by the outermost layer of cells, or epidermis. The epidermis corresponds to the ectoderm of Hydra. Beneath the epidermis lie two layers of muscle, an outer layer in which the fibres are arranged circularly round the body and an inner layer in which they run longitudinally. The chactae are lodged in small sacs, which are inpushings of the epidermis.

The Alimentary Canal

The mouth is a small crescent-shaped opening bordered by the peristomium and is overhung by the prostomium. It leads into a small chamber, the buccal cavity, which is followed by a larger chamber, the pharynx. From the pharynx the food passes into a short tube, the oesophagus, into which small glands forming a calcareous fluid pour their secretion. The oesophagus opens into a large, thin-walled chamber, termed the crop, which acts as a reservoir for the food before it is passed into the next region of the food canal, termed the gizzard. This is a thick-walled chamber with horny walls, leading into the intestine, which is a long straight tube running to the end of the body, where it opens to the exterior at the anus. The intestine is covered with a layer of yellow cells, and projecting into its cavity from the dorsal wall is a ridge called the typhlosole. The layer of cells which lines the inside of the intestine is called the endoderm, and corresponds to the endoderm of Hydra. The alimentary canal is supported and held in position by the septa.

The Blood System

The blood of the worm consists of a liquid, the plasma, which contains small colourless amoeboid cells. The plasma is coloured red by the pigment haemoglobin which is dissolved in it. The blood is carried in a complex system of vessels , one of which, the dorsal vessel, can be seen running as a red streak beneath the skin along the middle of the dorsal surface of the worm. This vessel lies on the upper side of the alimentary canal. Beneath the alimentary canal the ventral vessel runs along the whole length of the body, and is connected with the dorsal vessel in the region of the oesophagus by five pairs of contractile vessels lying in segments 7 to 11. Three smaller longitudinal vessels lie below the alimentary canal in close relation with the nerve cord. From the five main longitudinal vessels smaller branches ramify to the internal organs and to the body wall, which has a close network of blood capillaries lying beneath the epidermis.

The Excretory Organs

The excretory organs of the worm are called nephridia and occur in pairs in the coelom of each segment except the last and the first diree. Each nephridium is a long tube coiled in several loops and opening into the coelom at one end by a funnel-shaped opening termed the nephrostome. The nephrostome does not lie in the same segment as the ncphridium, but passes through the septum into the segment in front. The. nephrostome and part of the nephridium are lined with cilia, and the loops of the nephridium are surrounded by a network of blood vessels. The external opening lies between the two pairs of chffitae on each side.

The Nervous System , Ljing in the coelom beneath the alimentary canal is a slender t white thread termed the ventral nerve cord. Branch, nerves are given oil ’ from the nerve cord which pass to the skin and internal organs, and in each segment the cord shows a slight swelling termed ganglion. On the upper side of the buccal cavity are a pair of small white rounded bodies which are united in the middle line. These are the cerebral ganglia which form the ’brain ’of the worm, and. give off many small nerves to the prostomium. From the cerebral ganglia two bands of nervous tissue pass one on either side of the pharynx to join the anterior end of the ventral nerve cord, which lies beneath the pharynx.

The earthworm has no special sense organs and cannot see or hear. However, the front end of its body is sensitive to bright light, which the worm avoids. It has a well-developed sense of touch and is able to perceive vibrations in the soil. The worm has also a sense of smell, for it can recognize the presence of such foods as fat and meat. The sense cells which perceive these various stimuli are scattered in the epidermis, and each has a small sense hair which projects through the cuticle. The bases of the sense cells are connected with the ventral cord or cerebral ganglia by nerves.

Respiration

There are no special organs of respiration ; the worm respires through its thin damp skin in which lie many looped blood capillaries.

The Reproduction of the Earthworm

The earthworm is hermaphrodite and the male and female reproductive organs occur in segments 9 to 15. There are a pair of ovaries ’and two pairs of testes, the latter enclosed in large white hollow structures called the seminal vesicles in which the sperm formed by the testes develop and are stored. The eggs formed by the ovaries are stored in the swellings in the short oviducts until they pass out through their external openings on the ventral surface of the 14th segment. Two pairs of rounded sacs, the spermathecas, open to the exterior in the grooves between the 9th and 10th, and 10th and nth segments.

Although hermaphrodite, a single worm cannot fertilize its own eggs, but must pair with another worm in order that fertilization may take place. The spermatheca; store sperm received from the other worm during coition, when the sperm pass down the paired vasa defercntia to the male openings in the 15th segment.

The transference of sperm from one earthworm to another occurs during warm damp weather, usually during the early part of the day. Two animals lie in opposite directions and a secretion from their clitella fastens them together. Their hinder ends lie each in its burrow, so that the animals can retreat if disturbed. Seminal fluid containing sperm is then shed from the male openings of each worm and travels backwards by means of undulations in the floors of the seminal grooves to enter the spermathecae of the other partner where the sperm are stored. The animals separate and for the next week proceed to produce cocoons containing eggs and sperm.

During this process the clitellum secretes a thick band of substance and the animal then proceeds to slip backwards through it. A few eggs are shed into it through the apertures in segment 14, whilst later, little packets of sperm are shed when the cocoon passes segments 9 and 10. Epidermal cells also secrete a milky fluid into it. Eventually the animal slips its ’head ’out of the cocoon which, being elastic, closes up. Its front part is flattish whereas the other end is tapering, so that it is pear-shaped. The cocoon is usually left in a place where there is decayed vegetation. As it dries it becomes darker, tougher and somewhat horny. The eggs are fertilized by the sperm and, nourished by the milky fluid, develop into young like the parents. Sometimes three or four small worms will make their way out of a cocoon, but more commonly one develops at the expense of the others.

We may summarize by saying that the earthworm is hermaphrodite, exhibits copulation and cross-fertilization which, strictly speaking, is external.

Regeneration

Worms have considerable power of repairing injuries, for after a worm has been cleanly cut into head and tail parts by a spade, each half usually regrows the missing part, so that two worms result.

Practical Work on the Worm

Examine an earthworm and note the segments, chaitae, mouth, prostomium, clitellum, anus, dorsal blood vessel, and the two openings of the male genital ducts on the 15 th segment. Note the movements made by the worm when crawling. Draw the worm through the fingers in order to feel the chffitae. Place the animal on dry greaseproof paper to hear the chastae scratch it when they are pushed out.

Kill a worm with chloroform vapour or distilled water containing a little chloroform ; pin it down under the water and open the body cavity by a cut through the body wall in the mid-dorsal line. Note the septa, the regions of the alimentary canal, the dorsal blood vessel, the nephridia, the reproductive organs in segments 9 to 15. Push the intestine to one side and note the ventral blood vessel and the nerve cord and ganglia. Trace the nerve cord forward to the head and look for the circumpharyngeal commissures and the cerebral ganglia.

Remove a nephridium and mount in a drop of 1 per cent, salt solution on a slide. Examine under low and high powers of the microscope. Note the coiled tube of the nephridium and the lashing of the cilia which line part of the tube.

Examine a prepared slide of a transverse section through a worm. Use a lens or the low power of a microscope.

Examine a prepared slide of the cha^tae of the worm.

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